Final answer:
Isaac had one half-brother named Ishmael but no full brothers. The mention of Onan and laws of inheritance pertains to a different biblical story, not Isaac's family.
Step-by-step explanation:
Isaac, a patriarch in the Hebrew Bible, had only one brother, Ishmael. However, if we strictly adhere to the term 'full brother', Isaac had no full brothers; Ishmael was his half-brother, sharing the same father, Abraham, but different mothers. Isaac's mother was Sarah, and Ishmael's mother was Hagar. Therefore, answering the original question, Isaac did not have any brothers if excluding half-brothers.
The reference to Onan and the laws of inheritance mentioned in the context of your question is actually related to a different biblical story, which involves Judah's sons. It does not directly pertain to Isaac or his siblings.